Rare earth niobium oxynitrides, LnNbON2-δ (Ln = Y, La, Pr, Nd, Gd, Dy): Synthesis, structure and properties

Graphical abstract: Ammonolysis of LnNbO4 (Ln = rare earth or Y) leads to the formation of oxynitrides of different structures depending on the size of the rare earth. Highlights: → We have carried out ammonolysis of LnNbO4 (Ln = rare earth and Y) to form oxynitrides of different structures depending on the size of the rare earth. → Infrared spectroscopy shows the marked difference between oxides and the corresponding oxynitrides. → The oxynitrides are stable in air upto ∼400 oC above which an intermediate phase with nitrogen molecules attached to the oxide lattice forms. → Gadolinium niobium oxynitride shows paramagnetism. -- Abstract: Ammonolysis of rare earth niobates of the type LnNbO4 (Ln = Y, La, Pr, Nd, Gd, Dy) yields oxynitrides of different structures. When Ln = La, Nd and Pr, the structure is that of an orthorhombic perovskite of the general formula LnNbON2. As the size of the rare earth decreases, the oxynitride has a nitrogen-deficient defect fluorite (Ln = Pr, Nd, Gd), or pyrochlore (Ln = Y) structure. The IR spectra of the oxynitrides and the corresponding oxides are significantly different. Thermogravimetric analysis suggests the formation of an intermediate phase wherein the N2 molecule is attached to the oxide lattice above 400 oC and decomposes to give the oxide on heating in an oxygen atmosphere. Raman spectra of the intermediate phases show evidence for the N≡N stretching vibration. Gadolinium niobium oxynitride is found to be paramagnetic.